Proximate cause is the initial act which sets a natural and continuous series of events in motion that result in personal injury. If there was no initial act to create the injury, there would be no injury as a result.
Any action taken results in a serious of natural and continuous events. For example, if you have a ball in your hand, swing your arm, and release the ball and the ball rolls down a hill.
The last negligent act that results in the injury is where the responsibility lies. In the example of the ball rolling down a hill, if a person picks it up and throws it through a window, shattering it and causing a shard of glass to hit a person sitting next to the window, resulting in a cut that requires medical attention, the actions of the person who threw the ball, not the person who rolled it down the hill, were the proximate cause. The person who threw the ball threw the window would be held liable.